Merge pull request #4281 from rust-lang/state-option-etc

Ch. 18: correct discussion of delegation in `Post` methods

Author: chriskrycho

src/ch18-03-oo-design-patterns.md

@@ -359,11 +359,12 @@ known at compile time. (This is one of the dyn compatibility rules mentioned
 earlier.)
 
 Other duplication includes the similar implementations of the `request_review`
-and `approve` methods on `Post`. Both methods delegate to the implementation of
-the same method on the value in the `state` field of `Option` and set the new
-value of the `state` field to the result. If we had a lot of methods on `Post`
-that followed this pattern, we might consider defining a macro to eliminate the
-repetition (see [“Macros”][macros]<!-- ignore --> in Chapter 20).
+and `approve` methods on `Post`. Both methods use `Option::take` with the
+`state` field of `Post`, and if `state` is `Some`, they delegate to the wrapped
+value’s implementation of the same method and set the new value of the `state`
+field to the result. If we had a lot of methods on `Post` that followed this
+pattern, we might consider defining a macro to eliminate the repetition (see
+[“Macros”][macros]<!-- ignore --> in Chapter 20).
 
 By implementing the state pattern exactly as it’s defined for object-oriented
 languages, we’re not taking as full advantage of Rust’s strengths as we could.